The invention relates to a method for the continuous phase transformation of a product in a mixer-kneader into which the product is introduced, said mixer-kneader comprising kneading elements on at least one shaft.
Plug flow takes place in a continuous mixer-kneader, with a concentration profile over the length of the kneader and a homogeneous phase state for each axial position of the product. At the beginning of the kneader, after the entrance, for example, the material to be processed has a low viscosity since it contains a great deal of liquid, in particular solvent. Evaporation takes place only through contact with heat-exchange surfaces, these surfaces being limited as a rule to the inner wall of the kneader, the surface of the shaft and possibly also the surface of corresponding disk elements. Examples of such kneaders are described in DE 23 49 106 C, EP 0 517 068 A or DE 195 36 944 A. The list could be continued indefinitely.
EP 0 457 086 A2 describes a screw machine having means for the combined determination of the integral torque and of the torque input locally in certain sections of the product space. The measurement of the local moment provides information about the local torque input in the product space to the transported product. The local viscosity is also determined and can be calculated by means of the calculated torque curve and by means of at least one viscosity measurement. It provides information about the rheology properties of the product in the process space. The course of the chemical reactions in the process space is evaluated by means of the viscosity. It is also proposed to use the viscosity as a controlled variable, but this is effected by changing the viscosity according to the desired product qualities.
DE 100 08 531 A discloses a method for the continuous evaporation or for the thermal treatment of viscous products, in particular elastomers and thermoplastics, in a mixer-kneader comprising at least one stirring shaft equipped with mixing or kneading elements, fresh low-viscosity product solution being mixed into a preconcentrated, viscous product bed so that the viscosity and concentration of this product bed is adjusted so that the energy input, consisting of mechanical kneading energy and heat transfer via contact with the kneader heat-exchange surface, is maximum. It is also intended, inter alia, that the product be continuously back-mixed in the mixer-kneader. In this case, the main proportion of solvent (monomer, liquid) is evaporated owing to the kneading energy.
The heat transfer coefficient of such heat-exchange surfaces is low, especially if, for example, a foam forms between material and heat-exchange surface, and the specific heating area is unfavorable.
The evaporation then starts to become more efficient if the material is more viscous, if, for example, the proportion of solvent decreases below 20%. It now becomes easier to introduce the necessary kneading energy into this more pasty material, which energy is converted into heat. The question therefore arises as to the procedure to be adopted in a mixer-kneader so that the proportion of solvent is reduced as quickly as possible to a range in which the introduction of kneading energy becomes efficient.
It is the object of the present invention once again substantially to improve the mode of operation of the mixer-kneader and to make it more efficient.